One known spark plug includes an insulator that internally holds a center electrode and a metallic shell that internally holds the insulator. In such a spark plug, a sheet packing is held between the insulator and the metallic shell in order to ensure air tightness therebetween (see, for example, WO 2011/125306).
When the temperature of the center electrode of the spark plug is excessively high (e.g., 950° C. or higher), pre-ignition occurs in which the center electrode serves as a heat source and causes ignition to occur before spark discharge is generated. In a spark plug, its heat range (heat dissipation properties), which is the degree of dissipation of heat which the center electrode receives as a result of combustion to its surroundings, has been adjusted in order to prevent pre-ignition. One path for heat dissipation from the center electrode is a path extending from the insulator holding the center electrode through the sheet packing to the metallic shell. The heat of the metallic shell is released to the cylinder head of the internal combustion engine to which the spark plug is mounted.
In recent years, to achieve an improvement in the output power of an internal combustion engine and an improvement in its fuel economy simultaneously, there is a need for an increase in the set temperature of the combustion chamber. From the viewpoint of increasing the design flexibility of the internal combustion engine, there is a need for a reduction in the size of the spark plug. Under these circumstances, heat resulting from combustion tends to be accumulated in the spark plug.
In the spark plug in WO 2011/125306, there are no sufficient studies on how to dissipate heat sufficiently through a path extending from the insulator through the sheet packing to the metallic shell.